Topward 6303D - 25V max

[Mr_Chupon]

Hello,

I'm trying to debug a broken Topward 6303D. The maximum voltage that I can reach on one of the channel is about 25V at which point the power supply start draining 0.09A.

On the other channel, the relay clicks at about 25V and then I can reach 30V no problem.

This is only my first steps in debugging a power supply and my first intuition would be to simply change the relays since it doesn't seems to change state.

Could you guys give me a couple pointer that I could check to debug why the power supply cannot reach 30V?

Thank you

[Mr_Chupon]

Hello,

I've been trying to find the problem of this and I think I found the relay that doesn't click it's L6.

On the channel that works I can hear 3 clicks. One at about 7.5V, one at 16V and one at 24.5V. On the channel that doesn't work, I only hear one click at 7.5V which I guess is L7.

I was wondering how to make sure this is the problem. I was thinking of desoldering L6 and manually switch between 18V and 32.5V.

Could someone suggest me a way to debug this problem?

Thank you

*** Made a little mistake here it's not L6 but L2.

[Mr_Chupon]

Little update:

I don't think it's the relay the problem anymore. I got the old oscilloscope out and started probing around. It seems that Q3 is not responding like the transistor on the working channel. I get only 20V max on the base of Q3. On the other channel it goes beyond that. So my guess is that Q2 doesn't turn on so the relay is never switched.

So I guess the problem lies in the rest of the circuit. Unfortunately it's a little more involved than I would of liked.

So the next part is to reverse engineer all the 741 functions and check the regulator 78L12.

Help would be appreciated if you guys ever saw similar circuits or part of it. For examples the potentiometer VR1, VR2.

[Shock]

Can you post the full schematic or all the full pages you have?

After the channel switches the transformer taps at the relays it goes to the rectifier circuit and the filtering cap (i.e. C22), you want to monitor voltage there for a start to see if you are getting a comparable voltage level between channels.

The idea would be to see what minimal voltage can supply current (without issue) on the defective channel to narrow it down.

As for comparing the transistors, measure each collector/base/emitter voltage (channels set the same) and if unsure show where and what you are measuring then include it on an edited schematic in a post here.

If you suspect a specific transistor due to the voltages seen, test it in circuit then pull it out and test out of circuit.